This invention relates to a method for improving the capacity of a GSM base station in accordance with the preamble of claim 1, and a device to embody the method.
The present invention relates generally to the field of cellular radio communication, and in particular to base stations in a radio communication network.
The usage of mobile radio telephony has increased enormously during the past decade. As the load on the existing mobile radio systems is continuously increasing, it becomes more and more important to efficiently utilise the scarce frequency resources. A lot of effort is today put into optimising, given a certain amount of bandwidth, the number of connections with acceptable channel quality.
One way of improving the capacity of a mobile radio network is to introduce so called frequency hopping. When using frequency hopping in a network, the frequency used for transmission on a certain connection is changed at regular intervals. This results in an increased quality of the radio connection due to both the frequency diversity and the interference diversity hereby obtained (see e.g. H. Olofsson et al. xe2x80x9cInterference diversity as means for increased capacity in GSMxe2x80x9d, published EMPCC ""95). Frequency hopping is used mainly because radio signals are subjected to multi-path fading, which is space and frequency selective, but also to avoid interference with strong signals from neighbour cells transmitting on or close to the actual carrier frequency. Since the quality of the ongoing connections is generally increased as a result of the introduction of frequency hopping, a quality decrease caused by an increased number of connections in the system can be accepted. Thus, the quality increase can be traded for increased capacity.
Frequency hopping is introduced to a system by assigning to each connection a frequency hopping sequence that defines which frequency the connection will use at different points in time. Such a frequency hopping sequence may be defined by two different parameters:
a hopping sequence number, which defines the hopping sequence according to how the frequencies will vary, and
a frequency offset number, which defines where in the hopping sequence the connection in question will be at a particular point in time (see e.g. Global System for Mobile Communication (GSM) Technical Specification 05.02).
In GSM, all transceivers in a cell are assigned the same hopping sequence number, while each transceiver in a cell is assigned a cell unique frequency offset number.
Hereby is achieved that a connection will not experience any co-channel interference from other connections within the same cell. It has been shown that in order to achieve the desirable interference diversity gain by introducing frequency hopping, the number of frequencies to hop between should be at least three or four. In many situations there are not that many frequencies available for each cell. This problem can be solved by applying one of at least two different methods (see T. Toftegxc3xa5rd Nielsen et al., xe2x80x9cSlow frequency hopping solutions for GSM networks of small bandwidthxe2x80x9d, published VTC ""98).
1. The first method: By letting neighbouring cells form a pool of the frequencies allocated to each cell, where each transceiver being a member of the pool, utilises all frequencies available in the pool, the number of frequencies available for frequency hopping increases for each cell. To avoid co-channel interference within these neighbouring cells, the same hopping sequence is applied to each cell, but a unique frequency offset number is assigned to each transceiver. This solution, however, requires, that the cells are synchronised with each other.
2. The second method: The number of frequencies available for frequency hopping could be increased by using a small frequency reuse distance and thus obtain a high number of frequencies in each cell. In order to avoid an unacceptable level of co-channel interference the load on each frequency has to be limited.
Another way of increasing the capacity in a mobile radio network is to introduce so called adaptive antennas. Conventional antennas, which have an antenna lobe form which is static, are replaced by adaptive antennas, which can vary the form of the antenna lobe as well as the direction in which the antenna lobe is transmitted. This is provided by having an antenna array having a xc2xd wavelength distance between each other co-operating to form a lobe if different signal shifts are provided on the different antenna elements adapted to the wished lobe-form and lobe-direction.
A narrow antenna lobe can thus be directed towards the particular mobile station, which the base station is presently serving, instead of having an antenna lobe which covers the entire cell as is the case when a conventional antenna is used. Hereby is achieved that the overall interference level in the system is reduced, since each base station transceiver on the down-link transmits with narrow lobes in more concentrated geographical regions. Each base station receiver on the up-link rejects signals from other directions than the direction that it is presently configured for. For more detailed information regarding adaptive antennas, see e.g. S. Anderson et al. xe2x80x9cAdaptive Antennas for GSM and TDMA Systemsxe2x80x9d, published IEEE Personal Communications June 1999.
The effects of combining the use of adaptive antennas and frequency hopping in the same network has been investigated by F. Kronestedt et al. in xe2x80x9cAdaptive Antennas in Frequency hopping GSMxe2x80x9d, published ICUPC 1998. It has been found that in such networks the frequency reuse plan can be very tight. As mentioned in the CONCLUSIONS in the publication it is possible to carry full load in a ⅓ cell reuse case without DTX (Discontinuous Transmission) or power control. Even a {fraction (1/1)} cell reuse (all frequencies are used in every cell) might be possible as long as the channel utilisation is kept below 70%.
The problem discussed above of having too few frequencies available for hopping in each cell does not appear if the channel utilisation is kept on a low level. However, a limitation may be set for adjacent channel interference effects. With a tight reuse, such as ⅓ or {fraction (1/1)}, adjacent frequencies may be assigned to a cell. Thus, adjacent frequencies may be used simultaneously in a cell and adjacent interference from own cell will occur. This is very severe since the interfering signal (the adjacent interference) arises from the same base station as the desired signal.
In GSM, the frequency hopping procedure is described by two parameters in combination, i.e. MAIO (Mobile Allocation Index Office) and HSN (Hopping Sequence Number). In a cell each transceiver (TRX) is assigned the same HSN as the other transceivers in the cell, but a unique MAIO. The table below shows an example of this procedure for a cell A:
In this way, two transceivers in a cell will never use the same frequency simultaneously. Further, also exemplified in the table above, allocating MAIO in such a way that its value increments by at least 2 between TRXs in a cell results in that adjacent channel interference from the own cell is completely avoided. Adjacent frequencies are never used simultaneously.
If two TRXs in a cell have consecutive MAIO, adjacent channel interference will occur at every burst. This undesired behaviour would occur in the example below for a cell B:
This implies that consecutive MAIOs for a cell could not be used in the Prior Art networks. However, a drawback with the state of the art MAIO is, that allocation is only possible as long as the number of frequencies in the hopping sequence is twice as many as the number of installed transceivers in the cell. The reason is that the MAIO can only take as many values as the number of frequencies in the hopping sequence. In order to avoid adjacent channel interference only every second MAIO can be utilised. This criterion for Prior Art networks with conventional antenna techniques can be expressed in another way: the traffic load, defined as the number of TRXs per cell divided with the number of hopping frequencies per cell, must be below 50%, and in the reality lower than that in order to avoid interference from other cells.
However, simulations have shown that it could be possible for networks equipped with adaptive antennas to support traffic loads of up to 80 to 100%.
A problem to be solved according to the invention is thus to provide a method and/or a device to make it possible to use adjacent frequencies in one cell and be able to carry full load, or at least traffic load above 50% and still avoid adjacent cannel interference.
The solution according to the invention is to take the different antenna lobes and the MAIO used for each user into account. The general MAIO allocation rule is to make sure that mobiles with antenna lobes which can disturb each other severely are allocated MAIO incremented by at least 2, i.e. providing no adjacent channel interference. This may occur when users are located in the same direction or in directions very near to each other. However, if the users are separated such that they are seated within lobes with high suppression between each other they can be allocated MAIO incremented by 1.
Thus, the invention relates to a method and a device for making it able to support traffic loads to a very high extent for a cell of a radio base station in a cellular radio communication network in communication with mobile stations within the range of the radio base station. Frequency hopping and adaptive antenna means is used to provide at least two cell regions. A hopping list (HSN) of frequencies for the cell is allocated, and the frequency offset numbers (MAIOs) in the hopping list, to set the allowable frequency hopping procedure of the cell. The position in the cell for each mobile station within the cell boundary is determined. A frequency offset number (MAIO) is allocated for each mobile station. The cell is provided with at least one different lobe for each said cell region using a different adaptive antenna lobe for each said cell region. The lobes in the cell have suppression between each other. A different set of the frequency offset numbers (MAIOs) is assigned for each region. Each set for each of the cell regions has a value increment by at least two, and so that different regions have different sets of the frequency offset numbers (MAIOs) not overlapping each other.
The frequency offset numbers (MAIOs) to be used in a whole cell could have a value increment by one. Preferably, the adaptive antenna lobes are predetermined and fixed. If two cell regions then are provided, each defined by at least one lobe, allocating the frequency offset numbers (MAIOs) for one of the regions the values 1, 3, 5, etc. and the frequency offset numbers (MAIOs) for the other one of the regions the values 2, 4, 6, etc. could be the preferred one. If there are at least two lobes per region where the neighbouring lobes partly overlap each other, for the lobes nearest to a border line between the two regions, the lobe in one of the regions is allocated the smallest MAIO values in that region. The lobe in the other region is allocated the highest MAIO values in that region.
An intra-cell hand-over could be performed when one of the mobile stations moves from one of said cell regions to another. The intra-cell hand-over comprises then at least a change of the frequency offset number (MAIO) assigned to the radio connection in the cell region, from which the mobile station is moving, into a frequency offset number from the set of frequency offset numbers allocated to the cell region, to which the mobile station is moving.
The invention minimises the adjacent channel interference from the own cell (within cell) when using adaptive antennas in ⅓ and {fraction (1/1)} reuse cases. This implies improved quality and/or capacity.